>My question:Can the laser pointer be on 24h/day? Any suggestions?


Assuming you know the wheel size, at the height that the beam is, then
you can do this.
You'd be better off to use two beams, and measure the time between
breaking beam A and beam B.
That removes the size of the object as a variable.

Narrow beams have problems with bugs, rain, birds, and heat lensing.
You might think about a simple lens system to widen the beam to an
inch or two, then re-condense it on the other end.  This makes it
impossible for small things to block the beam.

You might try an alternative, grab a cheap microwave burglar alarm,
and take the speed from the Doppler output.